The Manas River (pron: [ˈmʌnəs]), known in Bhutan as the Drangme Chhu, is a transboundary river in the Himalayan foothills between southern Bhutan, India, and China. It is the largest of Bhutan's four major river systems, with the other three being the Amo Chu or Torsa River, the Wang Chu or Raidak, and the Puna Tshang Chu or Sankosh. It is met by three other major streams before it again debouches into India in western Assam. The river flows for a total length of 400 kilometres (250 mi), including through Tibet for 24 km (15 mi), Bhutan for 272 kilometres (169 mi), and Assam for 104 kilometres (65 mi) before it joins the mighty Brahmaputra River at Jogighopa. A major tributary of the Manas, the Aie River, joins it in Assam at Bangpari.

The Manas river valley has two major forest reserves, namely Royal Manas National Park (43,854 hectares (108,370 acres), established in 1966) in Bhutan and the contiguous Manas Wildlife Sanctuary (established at 391,000 hectares (970,000 acres) in 1955 and increased to 95,000 hectares (230,000 acres) in December 1985), encompassing a Project Tiger reserve, an elephant reserve and a biosphere reserve, which constitutes a UNESCO World Heritage Site declared in December 1985.

The Manas River drains 41,350 square kilometres (15,970 sq mi) of eastern Bhutan and northeast India. It has three major branches: the Drangme Chhu, Mangde Chhu, and Bumthang (Chamkhar) Chhu that cover most of eastern Bhutan, with the Trongsa and Bumthang valley also forming part of its catchment. The area drained in Bhutan territory is 18,300 km² and is bounded by the geographical coordinates 26°13′01″N 90°37′59″E / 26.217°N 90.633°E / 26.217; 90.633. A part of the main stem of the river rises in southern Tibet at an altitude of 4,500 metres (14,800 ft) before entering India in the Tawang district at the northwestern corner of Arunachal Pradesh.

The river flows through V-shaped gorges in a southwesterly direction between two ranges of the Lower Himalayas in Bhutan and enters Assam in the south-central foothills of the Himalayas. The valley widens in the foothills, where it is marked by the formation of swamps and marshes in the river's alluvial plain. The upper catchment is largely snowbound while the middle and lower catchment are thickly forested.

The total combined length of all tributaries of the river system in Bhutan amounts to 3,200 kilometres (2,000 mi). The main stem of the river, the Tawang Chu, originates in the Tawang district of Arunachal Pradesh in India and after flowing in a southwesterly direction enters Bhutan near Trashigang. At Duksum in Trashi Yangtse, the Tawang Chu is joined by the Kholong Chu, which rises in the northern Himalaya snow ranges of Bhutan. Further downstream, the Gamri Chu, flowing down from Sakten, joins it. At Trashigang, the width of the riverbed is about 550 metres (1,800 ft) and the riverbed elevation is 606 metres (1,988 ft).

The Lhobrak, or Kuri Chhu, is the main central tributary of the Manas. It is the only river that rises north of the Great Himalayas and it joins the Manas at Gongri Zomsa between Monggar and Pema Gatshel districts. Further downstream, at Panbang in Zhemgang District, the combined stream joins with the Mangde Chhu. Here the riverbed elevation is 121 metres (397 ft). The Mangde Chu is formed by two rivers, namely the Mangde Chu, which rises in northern Bhutan near Kula Kangri Peak where the elevation is 5,666 metres (18,589 ft), and the Bumthang River, also called the Chamkhar Chu.

After flowing in a generally southwesterly direction for about 29 kilometres (18 mi) in Bhutan, the Aie River, also known as the Mow River in Sarpang District in Bhutan, joins the Manas in the Goalpara District of Assam at the village of Agrong. From here, it follows a meandering course for about 75 kilometres (47 mi) and empties into the Brahmaputra near Jogighopa. The Aie River, which rises in the Black Mountains at an altitude of about 4,915 metres (16,125 ft) near the village of Bangpari, is about 110 kilometres (68 mi) in length. The total length of the Manas, as measured along its longest tributary the Kur, is 376 kilometres (234 mi), of which about 104 kilometres (65 mi) lies in India. About 270 kilometres (170 mi) of this length are in hilly or mountainous topography, with the balance in the plains. The Manas River is the largest north bank tributary of the Brahmaputra, with a recorded maximum discharge of 7,641 cubic metres, and contributes 5.48% of the total average volume of the Brahmaputra.

The river valley in the foothills is surrounded by small meadows located among thickly deciduous forested foothills with many rivulets, streams and natural drainage channels related to the river system. In the lower reaches of the river, there are many smooth sandy stretches populated with trees. The lowlands north of the confluence with the Brahmaputra, known as the Terai or Duars (a Sanskrit word meaning "passes" or "gates"), encompass a 15–30 kilometres (9.3–18.6 mi) stretch of the outermost foothills and are very fertile. Each Duar (with elevation varying from 100 metres (330 ft) to near sea level as they join the Brahmaputra) is named after a stream or hemmed between two streams. The fertile lands have been developed into tea estates and paddy fields.